1. Field of the Invention
This invention relates generally to planar antennas and, more particularly, to a multifunction, compact planar antenna that includes a finite superstrate having spatially configured air voids that control the variation of the effective dielectric constant of the superstrate across the antenna aperture to reduce or eliminate surface waves and/or standing waves in the superstrate, and thus power loss, and increase antenna performance.
2. Discussion of the Related Art
Current wireless communications systems, including radio frequency systems, global positioning systems (GPS), cellular telephone systems, personal communications systems (PCS), etc., typically require broadband antennas that are compact in size, low in weight and inexpensive to produce. Currently, radio frequency systems use the 20-400 MHz range, GPS use the 1-1.5 GHz range, cellular telephone systems use the 900 MHz range, and PCS use the 1800-2000 MHz range. The antennas receive and transmit electromagnetic signals at the frequency band of interest associated with the particular communications system in an effective manner to satisfy the required transmission and reception functions. Different communications systems require different antenna optimization parameters and design concerns to satisfy the performance expectations of the system.
The antennas necessary for the above-mentioned communications systems pose unique problems when implemented on a moving vehicle. The transmission and reception of electromagnetic waves into and out of a vehicle for different communications systems is generally accomplished through several antennas usually in the form of metallic masts protruding from the vehicle's body. However, mast antennas have significant drawbacks in this type of environment. In a typical design, the linear dimensions of a monopole mast antenna are directly proportional to the operational wavelength .lambda. of the system, and are usually a quarter wavelength for high performance purposes. Thus, at the lower end of the frequency spectrum, the size of a high-efficiency antenna becomes prohibitively large. For example, a monopole mast antenna used in the 800 MHz range should be around 10 cm long. Current military wireless communications systems use HF/UHF/VHF frequency bands, in addition to cellular telephone systems, GPS and PCS. For military communications in the 20 MHz range, the size of a high performance antenna is in the 4 m range. For military vehicles, mast antennas increase the vehicle's radar visibility, and thus reduce its survivability.
Further, when using multiple antennas to satisfy several communications systems, electromagnetic interference (EMI) between the antennas may become a problem. If the antennas are formed on a common substrate, the antenna signals tend to couple to each other and deteriorate the system's performance and signal-to-noise ratio. Thus, the design of multifunction antennas for military and commercial vehicles tends to pose major challenges with regard to the antenna size, radiation efficiency, fabrication costs, as well as other concerns.
To obviate the drawbacks of mast antennas, it is known in the art to employ planar antennas, including slot, microstrip, and aperture type designs, all well known in the art, for a variety of communications applications in the above-mentioned frequency bands, primarily due to the simplicity, conformability, low manufacturing costs and the availability of design and analysis software for such antenna designs. FIG. 1 shows a perspective view of a planar slot ring antenna 10 depicting this type of design, and is intended to represent all types of planar antenna designs. The ring antenna 10 includes a substrate 12 and a conductive metallized layer 14 printed on a top surface of the substrate 12. The layer 14 is patterned by a known patterning process to etch out a ring 16, and define a circular center antenna element 18 and an outside antenna element 20 on opposite sides of the ring 16. The antenna elements 18 and 20 are excited and generate currents by received electromagnetic radiation for reception purposes, or by a suitable transmission signal for transmission purposes, that create an electromagnetic field across the ring 16. A signal generator 22 is shown electrically connected to an antenna feed element 24 patterned on an opposite side of the substrate 12 from the layer 14. The signal generator 22 generates the signal for transmission purposes and receives the signal for reception purposes.
The antenna 10 is a slot antenna because no conductive plane is provided opposite to the layer 14. This allows the antenna 10 to operate with a relatively wide operational bandwidth compared to a metal-backed antenna configuration. However, the absence of a metallic ground plane results in radiation into both sides of the antenna, hence, bidirectional operation. In order to direct the radiation into one side of the antenna (unidirectionality), a high dielectric constant superstrate can be employed. FIG. 2 shows a cross-sectional view of the antenna 10 where a superstrate 26 having a high dielectric constant .di-elect cons..sub.r has been positioned on the layer 14, opposite to the substrate 12, to direct the radiation through the superstrate 26. The higher the dielectric constant .di-elect cons..sub.r of the superstrate 26, the more directional the antenna 10.
In addition to providing unidirectionality, a high dielectric constant superstrate also leads to antenna size reduction. The linear dimensions of planar antennas are directly proportional to the operational wavelength of the system. The transmission wavelength .lambda. of electromagnetic radiation propagating through a medium is determined by the relationship: ##EQU1## where C is the speed of light, f is the frequency of the radiation and .di-elect cons..sub.r is the relative dielectric constant or relative permittivity of the medium. For air, .di-elect cons..sub.r =1. In this context, the dielectric constant .di-elect cons..sub.r and the index of refraction n can be used interchangeably, since .di-elect cons..sub.r =n.sup.2. To significantly reduce the size of the antenna 10 for miniaturization purposes at a particular operational wavelength, it is known to position the superstrate 26 adjacent the layer 14 and make the superstrate 26 out of a high dielectric constant material, so that when the electromagnetic radiation travels through the superstrate 26, the wavelength is decreased in accordance with equation (1). This is because the guided wavelength along the antenna elements 18 and 20 is inversely proportional to the square root of the effective dielectric constant .di-elect cons..sub.eff, which in turn is related to the relative dielectric constant .di-elect cons..sub.r of the superstrate 26. The exact relationship depends on the particular geometry of the elements of the antenna 10. The dimensions of the antenna 10 would be well known to those skilled in the art for particular frequency bands of interest. By continually increasing the dielectric constant .di-elect cons..sub.r, the size of the antenna 10 can be further reduced for operation at a particular frequency band.
The use of a high dielectric constant superstrate is highly effective in reducing the size of the antenna so that it is practical for many high and low frequency communications applications. However, the use of high dielectric constant superstrates has a major drawback. It is known that planar antenna designs that employ high index substrates or superstrates have a significantly degraded performance due to the generation of surface waves and resonant or standing waves within the substrate or superstrate. These waves are generated because electromagnetic waves are reflected by dielectric interfaces, and are eventually trapped in the substrate 12 or superstrate 26 in the form of surface waves. The trapped waves carry a large amount of electromagnetic power along the interface and significantly reduce the radiated power from the antenna 10. The power carried by the excited surface waves is a function of the substrate characteristics, and increases with the dielectric constant of the substrate 12 or the superstrate 26. Additionally, the substrate 12 and/or superstrate 26 have the dimensions that cause standing waves within these layers as a result of resonance at the operational frequencies that also adversely affects the power output of the electromagnetic waves.
Consequently, an antenna printed on or covered by a high index material layer of the type described above, may have one or more of low efficiency, narrow bandwidth, degraded radiation pattern and undesired coupling between the various elements in array configurations. A few approaches have been suggested in the art to resolve the excitation of substrate modes in these types of materials, either by physical substrate alterations, or by the use of a spherical lens placed on the substrate 12. In all cases, the radiation efficiency is increased and antenna patterns are improved considerably as a result of the elimination of the surface wave propagation. However, all of these implementations have either resulted in non-monolithic designs or have been characterized by large volume and intolerable high costs.
The need to eliminate and/or reduce surface waves and standing waves in the superstrate region of a planar antenna of the type discussed above is critical for high antenna performance. To reduce these waves, it has been proposed by two of the inventors to replace the superstrate 26 with a planar superstrate having a graded index of refraction. The superstrate is formed from high index of refraction composite materials that are graded along one or both of the axial and radial directions. This concept is disclosed in provisional patent application 60/086701, filed May 26, 1998, titled "Multifunction Compact Planar Antenna With Planar Graded Index Superstrate Lens." By grading the dielectric constant of the superstrate 26 in one or both of the axial and radial directions, the electromagnetic waves propagating through the superstrate 26 encounter dielectric interfaces that alter the symmetry of the superstrate 26, and prevents the standing waves. Because of the lensing action of the superstrate 26, surface waves associated with traditional planar antennas printed on high index materials are suppressed causing the antenna efficiencies to increase dramatically.
FIGS. 3 and 4 depict this design by showing a cross-sectional view of the antenna system 10 that has been modified accordingly. In FIG. 3, the superstrate 26 has been replaced with a superstrate graded index lens 30 including three dielectric layers 32, 34 and 36 made from three materials with different dielectric constants so that the lens 30 is graded in the axial direction. The superstrate lens 30 is graded in a manner such that the layer 32 closest to the layer 14 has the highest dielectric constant, and the layer 36 farthest from the layer 14 has the lowest dielectric constant to gradually match the dielectric constant to free space. This design shows three separate dielectric layers 32-36 having different dielectric constants, but of course, more than three layers having different levels of grading can also be provided.
FIG. 4 shows a cross-sectional view of the antenna system 10 where the superstrate lens 26 has been replaced by a superstrate graded index lens 38 including three separate concentric dielectric sections 40, 42 and 44 having different dielectric constants to provide for grading in the radial direction. As above, three separate sections 40-44 are shown for illustration purposes, in that other sections having different dielectric constants can also be provided. With this design, the center section 40 has the highest dielectric constant and the outer section 44 has the lowest dielectric constant. In an alternate embodiment, the antenna system 10 can be graded in both the axial and radial directions in this manner. The lens material would be a suitable low-loss composite or thermally formed polymer. The lens 30 and 38 provide for size reduction of the antenna system 10, while providing high antenna performance by eliminating undesirable substrate modes. The radial grading of the lens would allow for the elimination of surface waves, while the axial grading would provide gradual matching of the antenna to free space to further enhance radiation efficiency.
The graded index superstrate lens design discussed above is effective for eliminating or reducing surface waves, but is limited in its operating frequency range because of current manufacturing capabilities of the lens. Particularly, the grading of the lens material is currently carried out using injection molding processes, where a composite material is injected into a host material with a varying volume fraction to achieve the desired permittivity profile. From an electrical point of view, this process introduces material losses, which become pronounced as the frequency increases. For a frequency range of interest covering FM radio bands through GPS and PCS (f&lt;2 GHz), the material processing technique is able to provide satisfactory performance. However, for higher frequencies at C-band or X-band and higher, providing the necessary material technology is out of reach at the present time. Also, the mechanical assembly of the graded index lens using machining and processing techniques have proven to be relatively costly and not amenable to mass production.
What is needed is a superstrate lens for a planar antenna that provides a varying effective dielectric constant profile across the lens to eliminate surface and standing waves for increased performance, but does not suffer from the limitations manufacturing referred to above. It is therefore an object of the present invention provide such a superstrate lens.